The present invention relates to a signal detector for detecting weak (or small) magnetic fields, electrical currents, voltage, electromagnetic wavers, etc. using a superconducting quantum interference device, which is a highly sensitivity magnetic sensor, and to a measuring method therefor, and more particularly relates to a broadband detector for measuring a high speed signal.
FIG. 6 shows an equivalent circuit diagram of a direct-current superconducting quantum interference device (hereafter referred to as a DC-SQUID). The DC-SQUID forms a superconducting ring using a SQUID inductance 2 and a pair of Josephson junctions 1 connected to the two ends of the SQUID inductance 2. A modulation coil 3 is magnetically coupled to the superconducting ring. An input coil 4 for inputting a signal is also magnetically coupled to the superconducting ring.
The DC-SQUID is a magnetic flux-voltage transducer for periodically converting voltage Vs on both ends of the superconducting ring to a single magnetic flux quantum ("PHgr"0: 2.07xc3x9710xe2x88x92‥Wb) for a magnetic flux "PHgr" mixed inside the superconducting ring. With a constant bias current supplied to the DC-SQUID, the magnetic flux-voltage characteristic is shown in FIG. 2A. As shown in FIG. 2A the DC-SQUID has high sensitivity to magnetic flux, but this response is non-linear and the dynamic range is narrow. In the case of low frequency measurement, in order to obtain sufficient linearity and timing a Flux locked loop (FLL) method, being a type of null-balance method, is used to drive the SQUID. However, the FLL method constructs a closed loop circuit, which means that with broadband over 1 MHz stable drive is difficult due to going out of lock etc. With broadband measurement, it is also possible to utilize a method where the SQUID is driven by an open loop circuit having a simple circuit structure.
FIG. 7 is a schematic diagram of a signal detector using a superconducting quantum interference device with an open loop circuit of the related art. This detector is comprised of a bias current supply circuit 6 for applying bias current I, a broadband amplifier circuit 7 for amplifying a voltage Vs of the DC-SQUID 5, and a modulation circuit 12 for moving operation points on the magnetic flux-voltage curve to given positions. The modulation circuit 12 supplies a modulation signal Im to the SQUID through a modulation coil 3. At that time, using the magnetic flux-voltage curve of FIG. 2A, the magnetic flux-voltage conversion coefficient is adjusted so as to be fixed at point A on a rapidly changing part of the graph. Voltage Vo that has been amplified by the amplifier circuit 7 is output as output voltage Vout, and the magnetic flux-voltage conversion coefficient, as well as an input signal from the mutual inductance between the input coil and the SQUID loop, are obtained.
In the case of using an open loop circuit, the structure of a drive circuit becomes simple, and high speed signal measurement becomes easy. However, due to external causes, such as environmental magnetic noise, source noise etc., there is movement of the operating points on the magnetic flux-voltage curve. Because of this movement of the operating points, there is a problem that the magnetic flux-voltage conversion coefficient required in obtaining the input signal from the output voltage also changes.
Also, since there is no linear region in the magnetic flux-voltage characteristic of the SQUID, there is a problem that it is not possible to accurately obtain the input signal from the output voltage.
A drive circuit for magnetic flux detection for reading out an input signal as an output voltage is made up of two systems, one being a flux-locked loop system for converting a voltage signal of the superconducting quantum interference device to a current signal and making magnetic flux inside the superconducting ring constant by feeding back a feedback signal to the modulation coil, and another being an output system for converting a voltage signal of the superconducting quantum interference device to a voltage signal, and not feeding back to the modulation coil but reading out the converted voltage signal as an output voltage.
Also, the frequency band of the flux-locked loop system is narrower than the frequency band of the output system.
The frequency band of the flux-locked loop system can also be controlled to take a given value. Further, a method of using the inventive device comprises the steps of measuring a flux-voltage curve of the superconducting quantum interference device before measuring an input signal, and converting an output signal obtained from the output system of the drive circuit into an input signal by comparison with the flux-voltage curve.